Tires are a complex structural combination of many components of various physical properties and of various compositions. Two of such components are a circumferential tire tread which is intended to be ground-contacting and pair of tire sidewalls which conventionally extend between the tire's spaced apart bead portions radially outward to the peripheral edges of the tire tread.
Tire treads and tire sidewalls are normally intended to present very different properties for a tire. For example, tire treads, which are intended to be ground contacting, are normally expected to provide physical properties such as, for example, traction and resistance to abrasion. For example, tire sidewalls, which are not intended to be ground contacting, are normally expected to provide physical properties such as, for example, resistance to flex fatigue and scuff resistance.
For this invention, it is desired to present a tire with a tread, and optionally its sidewalls, which contains silica reinforcement in which the silica reinforcement is provided as a pre-formed silica/coupler composite, namely a pre-hydrophobated precipitated silica.
In practice, various tires with silica reinforced rubber treads have been described and manufactured. Some tires have been mentioned which have silica containing rubber sidewall(s). Some tires have been mentioned which have both treads and associated sidewalls which are composed of silica containing rubber compositions.
Alkoxysilane-polysulfide based coupling agents (e.g. bis(3-triethoxysilylpropyl)polysulfides) are often used to aid in coupling of precipitated silicas to diene-based elastomers in a rubber composition. However, such ethoxysilane moieties rely upon reaction with hydroxyl groups (e.g. silanol groups) contained on the surface of the precipitated silica which in turn produces a volatile alcohol byproduct (e.g. ethanol) in situ within the rubber composition itself.
For this invention, it is desired to provide a tire with a tread of a rubber composition which has reduced, or substantially eliminated, volatile alcohol (e.g. ethanol) evaporative emission.
Accordingly, a tire is provided with a tread of a rubber composition which contains a pre-hydrophobated silica reinforcement and siloxane functionalized styrene/butadiene copolymer rubber.
For said silica pre-hydrophobation purposes, the precipitated silica may be pre-treated with a bis(3-triethoxysilylpropyl)polysulfide or alkoxyorganomercaptosilane, optionally including an alkoxysilane, prior to its addition to the tread rubber composition. The alkoxyorganomercaptosilane may alternately be in a form of a blocked alkoxyorganomercaptosilane. One aspect of such pre-hydrophobation of the precipitated silica is to reduce, or substantially eliminate, evolution of as volatile alcohol (ethanol) during the actual mixing of the silica with the rubber compositions in a rubber mixing operation.
From a historical perspective, preparation of tires have been suggested with a rubber component which contains a precipitated silica which is hydrophobated in situ within the elastomer host by addition of both a coupling agent, as an organomercaptosilane, and an alkyl silane. For example, see U.S. Pat. No. 4,474,908. However, for this invention, it is required that the precipitated silica is pre-hydrophobated to substantially reduce the hydroxyl group content on its surface prior to its addition to the rubber composition.
From a historical perspective, preparation of tires has also been suggested with a rubber component in which both a coupling agent, as an organosilyl polysulfide, and an alkylsilane are individually added to a silica-containing rubber composition to treat the amorphous silica in-situ within the rubber host with both a hydrophobating agent (the alkylsilane) and a silica coupler (the organosilyl polysulfide compound). For example, see U.S. Pat. No. 5,780,538.
From a historical perspective, preparation of tires has further been suggested where one or more of its components (e.g. tread and sidewall) are silica reinforced wherein the silica is pre-treated with a coupling agent and, optionally, a substituted alkylsilane, prior to addition of the silica to the associated rubber composition with an expected result of reduction of alcohol evolution upon mixing the silica with the rubber composition. For example, see European patent publication EP-A-1 142 730 where the silica is pre-hydrophobated with an alkylsilane (e.g. Formula I therein) such as, for example, an alkoxyorganosilane which will release an volatile alcohol byproduct (e.g. ethanol) upon reaction with alcohol groups (e.g. silanol groups) contained on the surface of a precipitated silica and an alkoxyorganomercaptosilane (e.g. Formula II therein) which may also release an volatile alcohol byproduct upon reaction with alcohol groups (e.g. silanol groups) contained on the surface of a precipitated silica. Various alternative blocked alkoxyorganomercaptosilanes has been suggested, for example, in PCT/US98/17391 and U.S. Pat. No. 3,692,812 patent publications.
Representative examples of such alkylsilanes, and particularly alkoxyalkylsilanes are provided in said European patent publication for the pre-hydrophobation of the silica (from its Formula I) are trichloro methyl silane, dichloro dimethyl silane, chloro trimethyl silane, trimethoxy methyl silane, dimethoxy dimethyl silane, methoxy trimethyl silane, trimethoxy propyl silane, trimethoxy octyl silane, trimethoxy hexadecyl silane, dimethoxy dipropyl silane, triethoxy methyl silane, triethoxy propyl silane, triethoxy octyl silane, and diethoxy dimethyl silane.
Representative examples of such alkoxyorganomercaptosilanes are mentioned in said European patent publication for the pre-hydrophobation of the silica (from its Formula (II) as triethoxy mercaptopropyl silane, trimethoxy mercaptopropyl silane, methyl dimethoxy mercaptopropyl silane, methyl diethoxy mercaptopropyl silane, dimethyl methoxy mercaptopropyl silane, triethoxy mercaptoethyl silane, and tripropoxy mercaptopropyl silane.
In practice, it is recognized that precipitated silica aggregates are typically hydrophilic (water attracting) in nature and, in order to aid in dispersing the silica aggregates in various rubber compositions, it is sometimes desired to make the silica aggregates more hydrophobic (water repelling) in nature and therefore more compatible with the rubber. Accordingly, and as described in the aforesaid U.S. Pat. Nos. 4,474,908 and 5,780,538, a hydrophobating agent may be added to a rubber composition in addition to the precipitated silica to combine with the silica in-situ within the rubber host to make the silica more hydrophobic in nature.
However, it is considered herein, for the tire tread application where properties are desired by silica reinforcement are often sought, and particularly where a good homogeneous dispersion in the rubber host is often sought, that an in-situ modification of the amorphous silica within a viscous rubber host on a hit and miss basis, under relatively harsh high sheer and high temperature conditions is a relatively inefficient procedure of modifying the amorphous silica for use in rubber compositions which are intended to be silica reinforced, particularly where both an organosilane and alkyl silane are used which would compete within the rubber composition for reaction sites on the silica surface and particularly where an alcohol byproduct is formed in situ within the rubber composition which is then available as an evaporative emission.
Accordingly in one aspect, it is proposed herein to provide a tire having a tread comprised of individual rubber compositions which contain particulate pre-hydrophobated precipitated silica aggregates where the silica aggregates are added to, or mixed with, the individual rubber compositions in a pre-hydrophobated form instead of more inefficiently subsequently hydrophobating the silica aggregates in situ within the elastomer host, thus essentially eliminating formation of a volatile alcohol byproduct (e.g. ethanol) therefrom in situ within the rubber composition itself and thereby essentially eliminating evaporative emission of such in situ formed volatile alcohol.
Historically, according to U.S. Pat. No. 5,750,610, an organosilicate-modified silica gel may be hydrophobated with both an organomercaptosilane and alkyl silane and the dried treated organosilicate-modified silica gel blended with natural rubber and/or synthetic rubber. This invention is not intended to be directed to use of such modified silica gels which, for the purposes of this invention, are intended to be distinguished from rubber reinforcing precipitated silica aggregates for this invention.
A general description of silica gel and precipitated silica may be found, for example, in the Encyclopedia of Chemical Technology, Fourth Edition (1997), Volume 21, Kirk-Othmer in Pages 1020 through 1023.
A further descriptive discussion of silica gels and precipitated silicas may be found, for example, in U.S. Pat. No. 5,094,829.
A significant aspect of this invention is the preparation of a tire with reduced volatile alcohol based evaporative emission which requires a tread component of a rubber composition which contains a combination of a siloxane functionalized elastomer and pre-hydrophobated precipitated silica aggregates, instead of using non pre-hydrophobated precipitated silica aggregates which are then hydrophobated in situ within the elastomer host, to produce a tire with reduced volatile alcohol evaporative emissions. It is considered that such tread composition is a departure from past practice, particularly where volatile alcohol based evaporative emission is taken into consideration, for a tire with acceptable performance characteristics.
In the description of this invention, the term “phr” relates to parts by weight for a material or ingredient per 100 parts by weight elastomer(s)”. The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.